How to prefetch video?
You'll find it in the Preview sidebar:

Shortcut 'P' is for setting the preview range, and will not influence the playback speed.
If you do not want to mess with proxies(which is the natural next step), try lowering the resolution percentage:

Nb. make sure that the project resolution is set to match your footage, by selecting a movie strip and in the menu select Movie Strip &gt; Set render size.